Implantable medical devices (IMD) for producing a therapeutic result in a patient are well known. An example of such an IMD includes implantable neurostimulators used for the treatment of movement disorders such as Parkinson's disease, essential tremor, and dystonia. Other examples of such IMDs include implantable drug infusion pumps, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators, and cochlear implants. It is recognized that other IMDs are envisioned that utilize energy delivered or transferred from an external device.
A common element in all of these IMDs is the need for electrical power in the IMD. The IMD requires electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse, or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an IMD can take one of two forms. The first form utilizes an external power source that delivers the energy via wires or radio frequency energy. Having electrical wires that perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such batteries usually do not supply the lasting power required to perform new therapies in newer IMDs. In some cases, such as an implantable artificial heart, a battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need for surgery to explant and re-implant the IMD or replace a portion of the device, such as the battery. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the IMD. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external antenna temporarily positioned on the surface of the skin and an external power source. Several systems and methods have been used for transcutaneous inductive recharging a rechargeable battery in an IMD.
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the IMD. The external coil, or primary coil, is associated with the external power source or external charger or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. The current can then be used to power the implanted medical device or to charge or recharge an internal power source or a combination of the two.
For RIMDs, the efficiency at which energy is transcutaneously transferred may be crucial. First, the inductive coupling, while inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer that can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging or recharging is occurring, the patient necessarily has an external encumbrance attached to his or her body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging or recharging can be accomplished thus limiting any inconvenience to the patient. Third, the amount of charging or recharging can be limited by the amount of time required for charging or recharging. Since the patient is typically inconvenienced during such charging or recharging, there is a practical limit on the amount of time during which charging or recharging should occur. Hence, the size of the internal power source can be limited by the amount of energy that can be transferred within the amount of charging time. It is evident that, the higher the efficiency of the energy transfer system, the greater amount of energy that can be transferred, and hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.
The problems with the external charging systems are that the external antenna must be aligned precisely with the implanted medical device in order to charge efficiently. For most external charging systems there is a connected LED display that communicates the charge rate so that the user may optimally position the external antenna. This process must be repeated each time the implanted device needs to be charged. Further once an optimal charging position is achieved it is hard to maintain this due to body movements. Even when the user is lying still, natural breathing movements may dislodge the external antenna from its optimum position. At present the available LED displays for the external charging systems do not warn a patient that optimal charging is not taking place. The patient must maintain visual contact with the display to make sure the device is continuing to charge optimally. What is needed in the art is a better system for maintaining the optimal position of the external antenna and better feedback for the user if the charger is not performing optimally.
U.S. Pat. No. 7,738,965 has attempted to address this issue with a holster that fits around the chest or waist of the user somewhat like a Sam Brown belt. A Sam Brown belt being a wide belt, which is supported by a strap passing diagonally over the right shoulder. This holster allows for a clip-on holder for an external charging device and contains a pocket that holds the antenna, one strap fits around the chest or waist while another goes over the shoulder. This holster is claimed by the inventors to be effective while the user is sitting upright in a chair performing tasks such as typing or writing. The present inventor has tested this device and found that this is not the case.
Therefore, need still exists for an apparatus and method for stabilizing an external antenna in an optimum position so as to limit amount of time needed to charge an implanted medical device. Optimally this apparatus would allow some level of activity during the charge so that the user is not substantially inconvenienced.
The present invention solves the problem of stabilizing an external antenna of the charger in the optimal position to recharge an implanted medical device by providing a unique molded cast to hold the external antenna and method of making this unique molded cast.